Megaupload reports "download limit exceeded". Given that I didn't download anything from Megaupload for at least a month now, this seems like a bug, but I don't really feel like debugging a crappy site like Megaupload...
Anyway, I fear I cannot help you much with corrupted saves. I tried to figure out several bugs from corrupted Nehrim saves, but there is no savegame reader for TES4 (like Enchanted Editor for TES3), so the only usable clues I can get are from yourself...
So, you say that #182 is not corrupted, but whenever you fire the cannons it becomes so? What happens if you DON'T fire the cannons, but just sit there, waiting for 3-4 days and saving again? Is the save corrupted once again?
If no, what if you just go on board, not yet firing the canons? Is it corrupted then?